Optical filters are important in numerous applications including both optical signal processing and optical communications applications. Likely uses for optical filters include, but are not limited to, wavelength division multiplexing, wavelength discrimination in frequency shift keying (FSK) coherent detection schemes as well as spontaneous emission noise filtering for otical amplifiers. As might be expected, several approaches have been taken in attempts to fabricate optical filters.
Waveguide reflection gratings provide one means of obtaining narrow band wavelength discrimination. This type of filter has been demonstrated in both glass and semiconductor waveguides. See, for example, Applied Physics Letters, 24, pp. 194-196, 1974, and Applied Physics Letters, 45, pp. 1278-1280, 1984. However, the drawbacks to this type of grating. It is difficult to obtain the desired narrow bandwidths. If the filter is made with strong coupling between the light and the grating, the light is reflected before seeing the entire garting, and the bandwidth is relatively large. If it is made with weak coupling between the light and grating, it must also be made long. In this case, the problems of obtaining both uniform grating and waveguide effective index become very difficult. In spite of these difficulties, filter bandwidth of approximately 6 Angstroms has been obtained for a center wavelength of .lambda.=1.55 .mu.m. It is often desired, however, for many applications that a filter bandwidth less than 1 Angstrom be obtained. Additionally, for some applications, a filter that works in transmission, rather than reflection, is desired.